The present invention relates to a pump unit used for various purposes.
More particularly, a first aspect of the present invention relates to a pump unit that includes a hydraulic pump, a pump case for accommodating the hydraulic pump and a center section connected to the pump case.
A hydraulic pump is used in various applications and in particular as the hydraulic pump adapted for operation in association with an actuator driven through the hydraulic effect. In this case, the hydraulic pump is connected to the actuator via a pair of hydraulic lines, and the output flow rate of hydraulic fluid discharged from the hydraulic pump is varied to cause the pressure difference between the pair of hydraulic lines, thereby driving the actuator. When the hydraulic pump is thus connected to the actuator via the pair of hydraulic lines so as to constitute a closed circuit, a charging mechanism is generally required to feed pressurized hydraulic fluid to the pair of hydraulic lines.
Specifically, the charging mechanism as required necessarily includes a charge line having a first end through which pressurized hydraulic fluid is fed into the pair of hydraulic lines, and a second end communicating with the pair of hydraulic lines, a checking valve for allowing the pressurized hydraulic fluid to flow from the charge line to the pair of hydraulic lines, while preventing the reverse flow.
For the hydraulic pump with the charging mechanism, it is desirable to reduce machining works for the reduction of the manufacturing cost, and/or improve an assembling efficiency in installing the check valve, or other works. However, there have not been made effective proposals in view of those points.
The first aspect of the present invention has been therefor conceived in consideration of the prior arts. It is an object of the first aspect of the present invention to provide a pump unit with the charging mechanism for feeding additional hydraulic fluid, which pump is used in association with the actuator driven through the hydraulic effect, and is capable of lowering the manufacturing cost and improving the assembling efficiency.
A second aspect of the present invention relates to a pump unit with first and second hydraulic pumps that is designed to be operated in association with an actuator driven through a hydraulic effect.
A hydraulic pump is used in various applications and in particular as the hydraulic pump adapted for operation in association with an actuator driven through the hydraulic effect. The description will hereinafter be made for the pump unit by taking for example the case where it includes the first and second hydraulic motors serving as the actuators that respectively drive the right and left drive wheels.
For example, U.S. Pat. No. 4,920,733 discloses a vehicle including first and second hydraulic pumps respectively connected via first and second hydraulic lines to the first and second hydraulic motors for driving the right and left drive wheels. In this vehicle, the first and second hydraulic motors respectively have outputs variable in response to the adjustment of the input/output flow rates of the first and second hydraulic pumps, thereby controlling the rotational speed and rotational direction of the right and left drive wheels.
The vehicle of the above arrangement has the first hydraulic pump and the second hydraulic pump separately arranged from one another, the former being operated in association with the first hydraulic motor, and the latter being operated in association with the second hydraulic motor. Such a separate arrangement of the hydraulic pumps involves a troublesome piping work between the first and second hydraulic pumps and the first and second hydraulic motors, a troublesome assembling work of the pump unit, and pose various other problems.
The second aspect of the present invention has been therefor conceived in consideration of the above prior art. It is an object of the second aspect of the present invention to provide a pump unit with the first and second hydraulic pumps that is capable of achieving the simplification of the piping work between the actuator and the hydraulic pumps, and the assembling work of the pump unit.
A third aspect of the present invention relates to a pump unit used for a vehicle with first and second hydraulic motors respectively connected to the right and left drive wheels, and includes first and second hydraulic pumps that are designed to be respectively operated in association with first and second hydraulic motors.
There are known arrangements of the above type described in, for example, U.S. Pat. No. 4,920,733. According to this U.S. patent, a vehicle with first and second hydraulic motors respectively connected to the right and left drive wheels includes first and second hydraulic pumps respectively operable in association with the first and second hydraulic motors. In this vehicle, the first and second hydraulic motors respectively have outputs variable in response to the adjustment of the input/output flow rates of the first and second hydraulic pumps, thereby controlling the rotational speed and rotational direction of the right and left drive wheels.
The vehicle of the above arrangement has the first hydraulic pump and the second hydraulic pump which are separately arranged from one another, the former being operated in association with the first hydraulic motor, and the latter being operated in association with the second hydraulic motor. Such a separate arrangement of the hydraulic pumps poses various problems, such as troublesome mounting operation of the hydraulic pumps on the vehicle, and troublesome assembling operation.
The third aspect of the present invention has been therefor conceived in consideration of the above prior art. It is an object of the third aspect of the present invention to provide a pump unit used for the vehicle with the first and second hydraulic motors respectively connected to the right and left drive wheels, and including first and second axial piston pumps of a variable displacement type that is capable of having improved efficiencies in mounting the pump unit on the vehicle, and improved assembling efficiency.
A fourth aspect of the present invention relates to a pump unit with first and second hydraulic pumps that are respectively connected via first and second hydraulic lines to first and second actuators driven through a hydraulic effect.
A hydraulic pump is used in various applications and in particular as the hydraulic pump adapted for operation in association with an actuator driven through the hydraulic effect. The description will hereinafter be made for the pump unit by taking for example the case where it includes the first and second hydraulic motors serving as the actuators that respectively drive the right and left drive wheels.
For example, U.S. Pat. No. 4,920,733 discloses a vehicle including first and second hydraulic pumps respectively connected via first and second hydraulic lines to the first and second hydraulic motors for driving the right and left drive wheels. In this vehicle, the first and second hydraulic motors respectively have outputs variable in response to the adjustment of the input/output flow rates of the first and second hydraulic pumps, thereby controlling the rotational speed and rotational direction of the right and left drive wheels.
The vehicle disclosed in the above cited U.S. patent has the first hydraulic pump and the second hydraulic pump separately arranged from one another, the former being operated in association with the first hydraulic motor, and the latter being operated in association with the second hydraulic motor. Such a separate arrangement of the hydraulic pumps invites a complicated structure of a feeding passage for feeding working hydraulic fluid from a reservoir tank to the first hydraulic line and the second hydraulic line, and poses various other problems.
The fourth aspect of the present invention has been therefor conceived in consideration of the above prior art. It is an object of the fourth aspect of the present invention to provide a pump unit with the first and second hydraulic pumps that are respectively connected via the first and second hydraulic lines to the first and second actuators driven through the hydraulic effect, and that is capable of achieving a simplified structure of the feeding passage for feeding working hydraulic fluid to the hydraulic lines extending between the actuators and the hydraulic pumps.
A fifth aspect of the present invention relates to a pump unit with first and second hydraulic pumps that are respectively connected via first and second hydraulic lines to first and second actuators driven through a hydraulic effect.
A hydraulic pump is used in various applications and in particular as the hydraulic pump adapted for operation in association with an actuator driven through the hydraulic effect. The description will hereinafter be made for the pump unit by taking for example the case where it includes the first and second hydraulic motors serving as the actuators that respectively drive the right and left drive wheels.
For example, U.S. Pat. No. 4,920,733 discloses a vehicle including first and second hydraulic pumps respectively connected via first and second hydraulic lines to the first and second hydraulic motors for driving the right and left drive wheels. In this vehicle, the first and second hydraulic motors respectively have outputs variable in response to the adjustment of the input/output flow rates of the first and second hydraulic pumps, thereby controlling the rotational speed and rotational direction of the right and left drive wheels.
The vehicle disclosed in the above cited U.S. patent has the first hydraulic pump and the second hydraulic pump separately arranged from one another, the former being operated in association with the first hydraulic motor, and the latter being operated in association with the second hydraulic motor. Such a separate arrangement of the hydraulic pumps invites a complicated structure of a feeding passage for feeding working hydraulic fluid for an HST (hydrostatic transmission) from a hydraulic fluid tank to the first hydraulic line and the second hydraulic line, and poses various other problems.
As a further disadvantage, the working hydraulic fluid between the hydraulic pumps and the actuators may increase in temperature due to the load from the outside. Such an increase in temperature of the working hydraulic fluid may invite various problems such as lowering of the volumetric efficiency, or lowering of the axle revolution speed if the hydraulic motors are used as the actuators for driving the drive wheels of the vehicle, deteriorating of the durability. However, the above-cited U.S. patent does not teach any solutions to limit the temperature of the working hydraulic fluid of the HST.
The fifth aspect of the present invention has been therefor conceived in consideration of the above prior art. It is an object of the fifth aspect of the present invention to provide a pump unit with the first and second hydraulic pumps that are respectively connected via the first and second hydraulic lines to the first and second actuators driven through the hydraulic effect, and that is capable of effectively limiting the increase in temperature of the working hydraulic fluid to be replenished to the hydraulic lines between the actuators and the hydraulic pumps.
According to the first aspect of the present invention, there is provided a pump unit that includes at least one hydraulic pump with inlet an outlet ports formed therein, a pump case for accommodating the at least one hydraulic pump, the pump case having an opening through which the at least one hydraulic pump is insertable into the pump case, and a lid (also referred to herein as a center section) connected to the pump case in such a manner as to close the opening of the pump case. The center section forms a pair of inlet/outlet passages having first ends respectively communicating with the inlet and outlet ports of the at least one hydraulic pump and second ends opening to the outside of the center section through a pump case abutting surface of the center section, and a first charging passage having a first end through which working hydraulic fluid is fed into the center section and a second end opening to the outside of the center section through the pump case abutting surface of the center section. At least one of the pump case and the center section forms a communication passage for communication between the second ends of the pair of inlet/outlet passages and the second end of the first charging passage. A first hydraulic fluid feeding valve is also provided in the pump unit for allowing hydraulic fluid to flow from the first charging passage to the pair of inlet/outlet passages while preventing the reverse flow. The first hydraulic fluid feeding valve is installable though the pump case abutting surface of the center section or a center section abutting surface of the pump case.
With the pump unit of the above arrangement, the workload for boring the center section can be reduced, and assembly efficiency in assembling the pump unit can be improved.
The pump case of the pump unit of the first aspect of the present invention is preferably adapted to reserve the hydraulic fluid.
The pump unit of the first aspect of the present invention preferably has the following arrangement. Specifically, the working hydraulic fluid fed into the first charging passage through the first end thereof is hydraulic fluid fed from a charge pump operatively connected to a driving shaft for driving the at least one hydraulic pump. The center section of the pump unit forms a second charging passage for communication between the inside of the pump case and the first charging passage; and the second charging passage preventing the flow of the hydraulic fluid from the first charging passage into the pump case, while allowing the hydraulic fluid stored within the pump case to flow into the first charging passage when negative pressure is generated in at least one of the pair of inlet/outlet passages,
The communication passage of the pump unit of the first aspect of the present invention preferably has a groove shape and formed in the center section abutting surface of the pump case. The pump case also has the center section abutting surface forming an escape groove communicating with the inside of the pump case for the escape of the leaked hydraulic fluid.
Further, the center section of the pump unit preferably forms a bypass passage for communication between the pair of inlet/outlet passages; and the bypass passage provided with an open/close valve in such a manner as to be operable from the outside for communication and cutoff of the hydraulic fluid between the pair of inlet/outlet passages.
According to the first aspect of the present invention, there is also provided a pump unit that includes a first hydraulic pump with inlet and outlet ports formed therein, a second hydraulic pump with inlet and outlet ports formed therein disposed parallel to the first hydraulic pump, and a pump case for accommodating the first hydraulic pump and the second hydraulic pump. The pump case has an opening through which the first hydraulic pump and the second hydraulic pump are insertable into the pump case. The center section is connected to the pump case in such a manner as to close the opening of the pump case. The center section forms a first pair of inlet/outlet passages having first ends respectively communicating with the inlet and outlet ports of the first hydraulic pump and second ends opening to the outside of the center section through a pump case abutting surface of the center section, a second pair of inlet/outlet passages having first ends respectively communicating with the inlet and outlet ports of the second hydraulic pump and second ends opening to the outside of the center section through the pump case abutting surface of the center section, and a first charging passage having a first end through which working hydraulic fluid is fed into the center section and a second end opening to the outside of the center section through the pump case abutting surface of the center section. At least one of the pump case and the center section forms a communication passage for communication the second ends of the first pair of inlet/outlet passages and the second ends of the pair of second inlet/outlet passages to the second end of the first charging passage. A first hydraulic fluid feeding valve is also provided in the pump unit for allowing the flow of hydraulic fluid from the first charging passage to the pair of inlet/outlet passage and the second pair of inlet/outlet passages. The first hydraulic fluid feeding valve is installable through the pump case abutting surface of the center section or a center section abutting surface of the pump case.
According to the second aspect of the present invention, there is provided a pump unit that includes a first hydraulic pump and a second hydraulic pump respectively connected to first and second actuators via a first pair of hydraulic lines and a second pair of hydraulic lines. The first hydraulic pump and the second hydraulic pump are accommodated within a common housing with the former pump disposed parallel to the latter pump. Both first and second hydraulic pumps are supported on a common center section. The common center section forms a first pair of inlet/outlet ports and a second pair of inlet/outlet ports. The first pair of inlet/outlet ports respectively serve as connection ports for connection with the first pair of inlet/outlet hydraulic lines, while the second pair of inlet/outlet ports respectively serve as connection ports for connection with the second pair of inlet/outlet hydraulic lines.
With the above arrangement, the conduit connection between the first and second actuators can be accomplished via the common center section, thereby achieving an improved efficiency in piping work. In addition, the first and second hydraulic pumps are accommodated within the common housing, so that the first and second hydraulic pumps can be installed on an object such as a vehicle through a single mounting operation.
The common center section of the pump unit of the second aspect of the present invention preferably forms a common charging passage for feeding pressurized hydraulic fluid into the first pair of hydraulic lines and the second pair of hydraulic lines. With this arrangement, the number of charging lines can be reduced as compared with the arrangement where the first and second hydraulic pumps are separately installed, resulting in a lower manufacturing cost.
The first and second pair of inlet/outlet ports of the second aspect of the present invention are preferably formed in the same side of the common center section, thereby achieving an improved efficiency in piping work between the first and second hydraulic motors.
The pump unit of the second aspect of the present invention preferably has the following arrangement. Specifically, the first hydraulic pump and the second hydraulic pump respectively have pump shafts connected together by a power transmission mechanism provided in the common housing. The common housing includes a partition wall through which the pump shafts of the first hydraulic pump and the second hydraulic pump can extend. The partition wall divides the common housing into a pump accommodation chamber and a power transmission mechanism accommodation chamber. With this arrangement, a single power transmission path is sufficient for the simultaneous rotation of the pump shafts of the first and second hydraulic pumps, resulting in a simplified structure of the power transmission mechanism for the power transmission from the power source to the pump unit. The partition wall can effectively prevent foreign matters such as iron powder generated in the power transmission mechanism from intruding into the pump accommodation chamber.
The pump unit of the second aspect of the present invention preferably has the following arrangement. Specifically, the first hydraulic pump and the second hydraulic pump are of an axial piston type that include angularly adjustable swash plates of a cradle type, respectively having rear sides forming spherical convex surfaces. The partition wall forms guiding surfaces respectively sized and shaped to slidingly guide the spherical convex surfaces of the angularly adjustable swash plates along the guiding surfaces. These surface formations can achieve lower manufacturing cost of the hydraulic pumps, and stabilized operation of the angularly adjustable swash plates.
According to the third aspect of the present invention, there is provided a pump unit used for a vehicle with first and second hydraulic motors respectively connected to the right and left drive wheels. Specifically, the pump unit includes a first hydraulic pump and a second hydraulic pump, both being of a variable displacement type located parallel to one another within a common housing, and respectively operable in association with the first and second hydraulic motors. The first hydraulic pump and the second hydraulic pump respectively include a first pump shaft and a second pump shaft located parallel to one another within the common housing and operatively connected to one another via a power transmission mechanism. The first and second control shafts are designed for controlling the input/output flow rates of the first and second hydraulic pumps. The first and second control shafts respectively extend away from one another along the vehicle width direction.
The pump unit of the above arrangement can achieve simplified mounting of the pump unit on the vehicle, and simplified structure for the power transmission between the power source and the pump unit. When the pump unit with the first and second control shafts extending away from one another along the vehicle width direction is mounted on a vehicle having push-pull control levers, the first and second control shafts can be disposed parallel to the push-pull type control levers, thereby achieving a simplified link mechanism between the control shafts and the control levers.
The first control shaft and the second control shaft of the third aspect of the present invention are preferably located substantially at the same position with respect to the vehicle longitudinal direction. This arrangement can achieve a more simplified link mechanism between the control shafts and the control levers.
The pump unit of the third aspect of the present invention preferably has the following arrangement. Specifically, the housing includes a partition wall between the first and second hydraulic pumps, and the power transmission mechanism, through which the first pump shaft and the second pump shaft can extend. The partition wall divides the housing into a hydraulic pump accommodation chamber and a power transmission mechanism accommodation chamber, thereby effectively preventing any foreign matters such as iron powder generated in the power transmission mechanism from adversely affecting the pump performance.
The pump unit of the third aspect of the present invention preferably has the following arrangement. Specifically, the first and second hydraulic pumps are of an axial piston type that respectively include angularly adjustable swash plates of a cradle type respectively having rear sides forming spherical convex surfaces. The partition wall forms guiding surfaces respectively sized and shaped to slidingly guide the spherical convex surfaces of the angularly adjustable swash plates. With this arrangement, the hydraulic pumps can be manufactured at low cost, and the angularly adjustable swash plates can be securely operated.
According to the fourth aspect of the present invention, there is provided a pump unit for operation in association with first and second actuators. The pump unit includes a first hydraulic pump and a second hydraulic pump respectively connected to the first and second actuators via a first pair of hydraulic lines and a second pair of hydraulic lines; a center section supporting the first hydraulic pump and the second hydraulic pump; a housing accommodating the first hydraulic pump and the second hydraulic pump. The first hydraulic pump, the second hydraulic pump, the first pair of hydraulic lines, the second pair of hydraulic lines, the center section and the housing are integrally connected together to constitute a single unit. The pump unit also includes a reservoir tank supportingly connected to the single unit for storing hydraulic fluid to be replenished to the first pair of hydraulic lines and the second pair of hydraulic lines.
The pump unit of the above arrangement can improve an efficiency in mounting the first and second hydraulic pumps on an object such as a vehicle, and shortening the length of the conduit for replenishing the hydraulic fluid from the reservoir tank to the first pair of hydraulic lines and the second pair of hydraulic lines, thereby lowering the manufacturing cost, and improving an efficiency in replenishing the hydraulic fluid through the decrease of the resistance force between the hydraulic fluid and the conduit wall, and producing other desirable effects
Preferably, the single unit of the pump unit of the fourth aspect of the present invention is designed so that the housing can serve as a hydraulic fluid tank, and the pump unit further includes a hydraulic fluid communication passage for providing a free fluid communication between the reservoir tank and the housing. With this arrangement, the number of the conduits required between the first and second hydraulic pumps, and the first and second actuators can be reduced to substantially four conduits only, specifically the first pair of hydraulic lines and the second pair of hydraulic lines. Thus, as compared with the conventional arrangements, the pump unit of this arrangement can achieve a lower manufacturing cost, an improved assembling efficiency and an excellent workability in maintenance. Since the housing itself also serves as a hydraulic fluid tank, the reservoir tank can compactly be made.
The pump unit of the forth aspect of the present invention preferably has the following arrangement. Specifically, the center section of the pump unit is a unitary member for supporting both first and second hydraulic pumps. The center section forms a first pair of hydraulic passages respectively having first ends communicating with the first hydraulic pump and second ends opening to the outside of the center section to form connection ports for connection with the first pair of hydraulic lines, a second pair of hydraulic passages respectively having first ends communicating with the second hydraulic pump and second ends opening to the outside of the center section to form connection ports for connection with the second pair of hydraulic lines, and a charging passage having a first end opening to the outside of the center section to form an inlet port for charging, serving as an inlet for the hydraulic fluid to be replenished and a second end communicating with the first pair of hydraulic passages and the second pair of hydraulic passages via check valves. The charging passage is connected to a pressure relief line communicating with the housing via a relief vale, and the inlet port for charging is connected to the reservoir tank via a hydraulic fluid replenishing passage.
The pump unit of the fourth aspect of the present invention also preferably has the following arrangement. Specifically, the center section includes a first center section and a second center section respectively supporting the first hydraulic pump and the second hydraulic pump. The first center section forms a first pair of hydraulic passages respectively having first ends communicating with the first hydraulic pump and second ends opening to the outside of the first center section to form connection ports for connection with the first pair of hydraulic lines. The second center section forms a second pair of hydraulic passages respectively having first ends communicating with the second hydraulic pump and second ends opening to the outside of the second center section to form connection ports for connection with the second pair of hydraulic lines. At least one of the first and second center sections forms a charging passage having a first end opening to the outside of the at least one of the first and second center sections to form an inlet port for charging, serving as an inlet for the hydraulic fluid to be replenished, and a second end communicating with the first pair of hydraulic passages and the second pair of hydraulic passages via check valves. The charging passage is connected to a pressure relief line communicating with the inside of the housing via a relief valve, and the inlet port for charging is connected to the reservoir tank via a hydraulic fluid replenishing passage.
The pump unit of the fourth aspect of the present invention also preferably has the following arrangement. Specifically, the pump unit includes a cooling fan provided near the single unit. The cooling fan is adapted to be driven in synchronism with the first and second hydraulic pumps. The reservoir tank is connected to the single unit in such a manner as to form a clearance therebetween, into which a cooling air stream is drawn from the cooling fan. The hydraulic fluid communication passage and the hydraulic fluid replenishing passage are disposed in such a manner to traverse the clearance. The thus arranged pump unit can limit the temperature increase of the hydraulic fluid stored in the reservoir tank and the housing, and also effectively limit the temperature increase of the hydraulic fluid flowing through the hydraulic fluid replenishing passage and the hydraulic fluid communication passage, thereby improving the transmission efficiency between the hydraulic pumps and the actuators.
According to the fifth aspect of the present invention, there is provided a pump unit for operation in association with first and second actuators. The pump unit includes: a first hydraulic pump and a second hydraulic pump respectively connected to the first and second actuators via a first pair of hydraulic lines and a second pair of hydraulic lines; a center section supporting the first hydraulic pump and the second hydraulic pump; and a housing accommodating the first hydraulic pump and the second hydraulic pump. The housing is adapted to be used as a hydraulic fluid tank. A hydraulic fluid circulation mechanism is also provided for taking the hydraulic fluid from the hydraulic tank, and again returning the same to the hydraulic tank. The hydraulic fluid circulation mechanism is designed to cool the hydraulic fluid while circulating the same.
The pump unit of the above arrangement can effectively limit the increase in temperature of the hydraulic fluid stored within the hydraulic tank, thereby effectively preventing deterioration in working efficiency of a hydraulic actuation device.
Preferably, the circulation mechanism of the pump unit of the fifth aspect of the present invention includes a circulation line, at least a portion of which is formed by a conduit; the circulation line having a first end communicating with the inside of the hydraulic tank and a second end again communicating with the inside of the hydraulic tank, and the conduit has at least a portion provided thereon with cooling fins.
The pump unit of the fifth aspect of the present invention preferably has the following arrangement. Specifically, the center section is a unitary member on which the first and second hydraulic pumps are supported in parallel relationship with one another. The housing forms an opening in a side thereof, through which the first and second hydraulic pumps can pass. The center section and the housing are integrally connected together to form a single unit, so that the opening of the housing can be sealed in a liquid tight manner by the center section with the first and second hydraulic pumps supported thereon. The center section forms a first pair of hydraulic passages respectively having first ends communicating with the first hydraulic pump and second ends opening to the outside of the center section to form connection ports for connection with the first pair of hydraulic lines, a second pair of hydraulic passages respectively having first ends communicating with the second hydraulic pump and second ends opening to the outside of the center section to form connection ports for connection with the second pair of hydraulic lines, and a charging passage having a first end communicating with the hydraulic fluid tank to form an inlet port for charging, serving as an inlet for the hydraulic fluid to be replenished and a second end communicating with the first pair of hydraulic passages and the second pair of hydraulic passages via check valves. The pump unit further comprises: a charge pump for sucking the hydraulic fluid stored within the hydraulic fluid tank and then discharging the same into the inlet port for charging; a pressure relief line having a first end connected to the charging passage via a relief valve and a second end forming a drain port through which the hydraulic fluid from the relief valve is drained; and a pipe connecting the second end of the pressure relief line with the hydraulic fluid tank; in which the pipe constitutes a conduit, and the charge pump constitutes a part of the hydraulic fluid circulation mechanism.
The pump unit of the fifth aspect of the present invention also preferably has the following arrangement. Specifically, the center section includes a first center section and a second center section respectively supporting the first and second hydraulic pumps. The housing has first and second sidewalls facing one another and respectively forming a first opening and a second opening through which the first hydraulic pump and the second hydraulic pump can respectively pass. The first and second center sections are integrally connected to the housing to form a single unit, so that the first and second openings of the housing are sealed in a liquid tight manner by the first and second center sections respectively supporting the first and second hydraulic pumps thereon. The first center section forms a first pair of hydraulic passages respectively having first ends communicating with the first hydraulic pump and second ends opening to the outside of the first center section to form connection ports for connection with the first pair of hydraulic lines. The second section forms a second pair of hydraulic passages respectively having first ends communicating with the second hydraulic pump and second ends opening to the outside of the second center section to form connection ports for connection with the second pair of hydraulic lines. At least one of the first and second center sections forms a charging passage having a first end communicating with the hydraulic fluid tank to form an inlet for the hydraulic fluid to be replenished and a second end communicating with the first pair of hydraulic passages and the second pair of hydraulic passages via check valves. The pump unit further comprises: a charge pump for sucking the hydraulic fluid stored within the hydraulic fluid tank and then discharging the same into the inlet port for charging; a pressure relief line having a first end connected to the charging passage via a relief valve and a second end forming a drain port through which the hydraulic fluid from the relief valve is drained; and a pipe connecting the second end of the pressure relief line with the hydraulic fluid tank; in which the pipe constitutes a conduit, and the charge pump constitutes a part of the hydraulic fluid circulation mechanism.
The pump unit of the fifth aspect of the present invention preferably includes a reservoir tank, in which the reservoir tank is in free fluid communication with the housing via a hydraulic fluid communication passage, and forms a hydraulic fluid tank in cooperation with the housing, in which the inlet port for charging communicates with the reservoir tank via a hydraulic fluid replenishing passage.
The pump unit of the fifth aspect of the present invention preferably includes cooling fins provided on the hydraulic fluid replenishing passage and the hydraulic fluid communication passage.
The pump unit of the fifth aspect of the present invention preferably has the following arrangement. Specifically, a cooling fan adapted to be driven in synchronism with the first and second hydraulic pumps is provided near the single unit. The reservoir tank is connected to the single unit in such a manner as to form a clearance therebetween, into which a cooling air stream from the cooling fan is drawn. The hydraulic fluid communication passage and the hydraulic fluid replenishing passage are disposed to transverse the clearance.
A cooling air duct is preferably provided in the pump unit of the fifth aspect of the present invention, so that a cooling air stream from the cooling fan is drawn into the clearance along the cooling air duct.